Or at Destructive Plate Boundaries 1 Convection currents

Or at Destructive Plate Boundaries…. . 1. Convection currents cool and descend causing two plates to be pulled towards each other( or by ‘slab pull theory’) Oceanic crust denser/heavier but thinner forced beneath lighter continental crust 5. Fold mountains form as sediments are crumpled up on leading edges of continental masses Continental Crust Oceanic Crust 2. Crust Melts in subduction zone and creates a deep ocean trench 3. Friction causes heat which melts ocean crust in to mantle in the BENIOFF ZONE which causes earthquakes (SEE NOTES. . ) 4. Could lead to rising magma (molten rock) – volcanic eruption

SUBDUCTION ZONE • Subduction zone - where two tectonic plates move towards one another subduction can occur. This involves an oceanic plate sliding beneath either a continental plate or another oceanic plate. • Subduction zones are often noted for their high rates of volcanism, earthquakes, and mountain building. This is because subduction processes result in melting of the mantle that produces a volcanic arc as relatively lighter rock is forcibly submerged

Shallow-focus and deep-focus earthquakes • The majority of tectonic earthquakes originate at the Pacific Ring of Fire in depths not exceeding tens of kilometres. Earthquakes occurring at a depth of less than 70 km are classified as 'shallow-focus' earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed 'mid-focus' or 'intermediatedepth' earthquakes. • In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deepfocus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers). These seismically active areas of subduction are known as ‘Benioff zones’ • Deep-focus earthquakes occur at a depth at which the subducted lithosphere should no longer be brittle, due to the high temperature and pressure.

What is the ‘Benioff zone’? • • (Ref Hodder p. 7) The BENIOFF ZONE is an area of seismicity corresponding with the slab being thrust downwards in a subduction zone. The different speeds and movements of rock at this point produce numerous earthquakes. It is the site of intermediate/ deep-focused earthquakes. This is an important factor in determining earthquake magnitude since it determines the position and depth of the ‘hypocentre’

What is the hypocentre? (Ref : Hodder p. 8 see fig 1. 10) • This is the ‘focus’ point within the ground where the strain energy of the earthquake stored in the rock is first released. • The distance between this and the epicentre on the surface is called the FOCAL LENGTH • The hypocentre can occur at any depth between the earth’s surface and about 700 km. • The most damaging events are usually shallow focus , with a hypocentre of less than 40 km

TASK: RESEARCH FAULTS https: //www. youtube. com/watch? v=q. Q 9 Mw_ rt. Dng&feature=player_embedded What is a normal fault? What is a strike/ slip fault? What is a thrust/ Megathrust fault and how is it caused? What is meant by the term a locked fault? (Ref : Hodder p. 7)

Faults : Another zone where earthquakes occur • Two sides of the fault slip horizontally past each other ; transform boundaries are a particular type of strike-slip fault • Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary /constructive • Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary/destructive

THRUST/MEGATHRUST/FAULTS • Thrust/Megathrust fault: • Formed at a subduction zone at the point of plate contact. When the plates are locked together, frictional stress builds. • When this stress exceeds a given threshold, a sudden failure occurs along the fault plane that can result in a ‘mega thrust earthquake’ e. g. Japan earthquake and tsunami March 2011 • This releases strain energy and radiates seismic waves

Diagram to show tectonic activity and cause of ‘mega thrust earthquake’ in Japan

LOCKED FAULT • It is common for the leading edge of a megathrust fault to LOCK under high friction. This locked fault may hold for thousands of years, building up enormous strains of energy before releasing. • The process of strain , stress and failure is referred to as the ‘elastic rebound theory’. • A locked fault is therefore one that is not slipping because the frictional resistance on the fault is greater than the shear stress across the fault- so it is stuck.

• Such faults may store strain for extended periods that is eventually released in a large magnitude earthquake when the frictional resistance is eventually overcome • E. g The 2004 Indian Ocean tsunami was the result of a megathrust locked fault(subducting Indian Plate)with strain energy building at @20 mm per year. It generated huge seismic waves and the devastating tsunami. https: //www. youtube. com/watch? v=FRY 3 f. NNY 30 • http: //www. bbc. co. uk/science/earth/natural_disa sters/earthquake#p 00 gfgjj

Diagram to show a tsunami forming as a result of strain energy being released from a locked fault

TASK ‘Explain the physical processes which would account for the reasons why some earthquakes are of low and some are of high hazard risk. ’ (10 marks) TIPS What is a hazard/hazard risk? Explain: give reasons why Physical processes: detail required at different plate boundaries Include key terms: e. g plate boundary types, subduction, Benioff zone , hypocentre…. Give examples (Ref Hodder p 4 -9)

Human activity can also cause earthquakes e. g. FRACKING FOR SHALE GAS (Lancashire UK-2011 present) So, are humans also responsible for earthquakes? See ‘The Times ‘ newspaper article ( synoptic angle. . )

Recap: Where do earthquakes happen? Are they ALL found at plate boundaries?

TASK: RESEARCH How are intraplate earthquakes formed? • https: //www. geolsoc. org. uk/Plate. Tectonics/Chap 3 -Plate-Margins/Midplate(Ref : The Geological Society) • http: //www. ck 12. org/earthscience/Intraplate. Earthquakes/lesson/Intraplate. Earthquakes-MS-ES/

INTRAPLATE EARTHQUAKES • @ 5% of earthquake activity takes place WITHIN a plate, AWAY from a plate boundary… • The causes of intra plate earthquakes is still not fully understood. Some are associated with pre existing weaknesses in plates which can become reactivated. • It is thought that zones of weaknesses occur as plates (pieces of solid lithosphere) move about on a spherical surface. Intra plate earthquakes therefore form along these lines of weaknesses.

• Whilst intraplate earthquakes are not usually as large as earthquakes found along plate boundaries they can still cause a large amount of damage………………. .

Intraplate earthquake: Gujarat, NW India 2001

Effects of intra plate Gujarat earthquake 2001

TASK • Research 5 facts (LITERALLY!) about the Gujarat India earthquake 2001 • https: //www. youtube. com/watch? v=Wk. AX Y 8 Dux 6 o •

H/W/CW • Make brief notes on the case study of the Market Rasen earthquake, Lincolnshire UK Feb 27 th 2008. • Why did it occur if the UK is not on a plate boundary?

See H/W Task ( separate yellow sheet) • Mystic Meg 2016? !. . • ‘With the use of examples, describe the characteristics and explain the causes of earthquakes. ’

TASK: Describe and explain the global distribution of earthquakes. Which areas are of particularly high risk?

PLENARY • Devise 3 factual questions ( with answers) from this lesson and test your neighbour! • 5 mins- GO

H/W • Learn all key terms for a quick points test next lesson